Hair growth is a cyclical process which includes a growth stage (anagen), a regression stage (catagen), and a quiescent stage (telogen). During anagen, the hair bulb within the follicle penetrates the dermis and contacts the dermal papilla, triggering division of hair matrix keratinocytes. The new keratinocytes dehydrate and condense to form the hair shaft, which is pushed through the epidermis by newly dividing keratinocytes in the hair root. Hair growth ends in the catagen phase. The hair bulb separates from the dermal papilla, retracts from the dermis, and the follicle shrinks in size. In telogen, the hair remains attached to the follicle but, due to its shallow position in the epidermis, can easily be released from the skin. Normally, the follicle transitions back into anagen phase, during which the hair is pushed out of the follicle by hair newly formed by dividing keratinocytes. Disruption of the hair growth cycle leads to thinning and baldness. On the scalp, hair follicles shrink and shed terminal (long, pigmented) hair. The lost hair is either not replaced by new hair or is replaced by vellus (thin, short, non-pigmented) hair, resulting in the appearance of baldness.
The most common pharmacotherapeutics currently used to treat hair loss are minoxidil and 5-alpha reductase inhibitors, such as finasteride. The precise mechanism by which minoxidil reduces hair loss is unknown; and there is a significant percentage of patients that do not respond to therapy. While finasteride has been shown to slow hair loss in men, the drug is associated with several side effects, including gynecomastia and sexual dysfunction. Both minoxidil and anti-androgens can require several weeks to increase hair count, and must be continued indefinitely on a daily basis to maintain effectiveness. DiColandrea et al. U.S. Pat. No. 8,986,664 describes one alternative approach in which certain monoamine oxidase inhibitors (MAOI) are administered together with a vasodilator to improve hair biology and hair growth.
There remains a need for more effective treatments for improving hair growth and/or for restoring hair color. It would be particularly desirable to development treatments which also improve other qualities of hair associated with aging, such as texture, body, and sheen, which may help provide an overall more youthful appearance.